make fallback skin configurable

[ossi]

selecting a 256-color skin merely yields a warning and a fallback to the default scheme when mc is run in a 16-color terminal (which the linux console is). this is rather suboptimal - the fallback should be explicit (and give no warning when selected).

note that the manual color configuration already has terminal-based selection logic.

i already made related comments on #2173.

#2737 proposes a rather less-than-beautiful workaround.

[zaytsev]

So what is the desired solution? Command-line flag like --fallback-skin ? What if the specified skin is also problematic, fallback to hardcoded default without warning (or with a warning)?

[ossi]

a command line flag would not be very user-friendly.

one way to treat it would be not as a fallback at all, but just have separate settings for 16m, 256, and 16 colors, which would be more or less consistent with the low-level color settings.

if it's a fallback, then a cascading behavior would make sense, and a warning when falling back to the builtin.
however, the UI for this would be more complex, just as the implementation overall. so i guess the first approach is preferable.